This invention relates to wideband code division multiple access (WCDMA) for a communication system and more particularly to circuit for switching between space time block coded transmit antenna diversity and non-diversity for WCDMA.
Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication systems as described in Provisional U.S. patent application Ser. No. 09/205,029, filed Dec. 3, 1998, and incorporated herein by reference. These WCDMA systems are coherent communications systems with pilot symbol assisted channel estimation schemes. These pilot symbols are transmitted as quadrature phase shift keyed (QPSK) known data in predetermined time frames to any receivers within range. The frames may propagate in a discontinuous transmission (DTX) mode. For voice traffic, transmission of user data occurs when the user speaks, but no data symbol transmission occurs when the user is silent. Similarly for packet data, the user data may be transmitted only when packets are ready to be sent. The frames include pilot symbols as well as other control symbols such as transmit power control (TPC) symbols and rate information (RI) symbols. These control symbols include multiple bits otherwise known as chips to distinguish them from data bits. The chip transmission time (TC), therefore, is equal to the symbol time rate (T) divided by the number of chips in the symbol (N).
Previous studies have shown that multiple transmit antennas may improve reception by increasing transmit diversity for narrow band communication systems. In their paper New Detection Schemes for Transmit Diversity with no Channel Estimation, Tarokh et al. describe such a transmit diversity scheme for a TDMA system. The same concept is described in A Simple Transmitter Diversity Technique for Wireless Communications by Alamouti. Tarokh et al. and Alamouti, however, fail to teach such a transmit diversity scheme for a WCDMA communication system.
Referring to FIG. 1, there is a simplified block diagram of a typical transmitter using Space Time Transit Diversity (STTD) of the prior art that is compatible with WCDMA. The transmitter circuit receives pilot symbols, TPC symbols, RI symbols and data symbols on leads 100, 102, 104 and 106, respectively. Each of the symbols is encoded by a respective STTD encoder as will be explained in detail. Each STTD encoder produces two output signals that are applied to multiplex circuit 120. The multiplex circuit 120 produces each encoded symbol in a respective symbol time of a frame. Thus, a serial sequence of symbols in each frame is simultaneously applied to each respective multiplier circuit 124 and 126. A channel orthogonal code Cm is multiplied by each symbol to provide a unique signal for a designated receiver. The STTD encoded frames are then applied to antennas 128 and 130 for transmission.
Turning now to FIG. 2, there is a block diagram showing signal flow in an STTD encoder of the prior art that may be used with the transmitter of FIG. 1. The STTD encoder receives symbol S1 at symbol time T and symbol S2 at symbol time 2T on lead 200. The STTD encoder produces symbol S1 on lead 204 and symbol xe2x88x92S2* on lead 206 at symbol time T, where the asterisk indicates a complex conjugate operation. Furthermore, the symbol time indicates a relative position within a transmit frame and not an absolute time. The STTD encoder then produces symbol S1 on lead 204 and symbol S1* on lead 206 at symbol time 2T. The bit or chip signals of these symbols are transmitted serially along respective paths 208 and 210. Rayleigh fading parameters are determined from channel estimates of pilot symbols transmitted from respective antennas at leads 204 and 208. For simplicity of analysis, a Rayleigh fading parameter xcex1j1 is assumed for a signal transmitted from the first antenna 204 along the jth path. Likewise, a Rayleigh fading parameter xcex1j2 is assumed for a signal transmitted from the second antenna 206 along the jth path. Each ith chip or bit signal rj(i+xcfx84j) of a respective symbol is subsequently received at a remote mobile antenna 212 after a transmit time xcfx84j corresponding to the jth path. The signals propagate to a despreader input circuit (FIG. 6) where they are summed over each respective symbol time to produce output signals Rj1 and Rj2 corresponding to the jth of L multiple signal paths as previously described.
Referring now to FIG. 3, there is a schematic diagram of a phase correction circuit of the prior art that may be used with a remote mobile receiver. This phase correction circuit receives signals Rj1 and Rj2 as input signals on leads 612 and 614 as shown in equations [1-2], respectively.                               R          j          1                =                                                            ∑                                  N                  -                  1                                                            i                =                0                                      ⁢                                          r                j                            ⁡                              (                                  i                  +                                      τ                    j                                                  )                                              =                                                    α                j                1                            ⁢                              S                1                                      -                                          α                j                2                            ⁢                              S                2                *                                                                        [1]                                          R          j          2                =                                                            ∑                                                      2                    ⁢                    N                                    -                  1                                                            i                =                N                                      ⁢                                          r                j                            ⁡                              (                                  i                  +                                      τ                    j                                                  )                                              =                                                    α                j                1                            ⁢                              S                1                                      +                                          α                j                2                            ⁢                              S                1                *                                                                        [2]            
The phase correction circuit receives a complex conjugate of a channel estimate of a Rayleigh fading parameter xcex1j1* corresponding to the first antenna on lead 302 and a channel estimate of another Rayleigh fading parameter xcex1j2 corresponding to the second antenna on lead 306. Complex conjugates of the input signals are produced by circuits 308 and 330 at leads 310 and 322, respectively. These input signals and their complex conjugates are multiplied by Rayleigh fading parameter estimate signals and summed as indicated to produce path-specific first and second symbol estimates at respective output leads 318 and 322 as in equations [3-4].
xe2x80x83Rj1xcex1j1*+Rj2*xcex1j2=(|xcex1j1|2+|xcex1j2|2)S1xe2x80x83xe2x80x83[3]
xe2x88x92Rj1*xcex1j2+Rj2xcex1j1*=(|xcex1j1|2+|xcex1j2|2)S2xe2x80x83xe2x80x83[4]
These path-specific symbol estimates are then applied to a rake combiner circuit to sum individual path-specific symbol estimates, thereby providing net soft symbols as in equations [5-6].                                           S            ~                    1                =                                                            ∑                L                                            j                =                1                                      ⁢                                          R                j                1                            ⁢                              α                j                                  1                  *                                                              +                                    R              j                              2                *                                      ⁢                          α              j              2                                                          [5]                                                      S            ~                    2                =                                                            ∑                L                                            j                =                1                                      ⁢                                          -                                  R                  j                                      1                    *                                                              ⁢                              α                j                2                                              +                                    R              j              2                        ⁢                          α              j                              1                *                                                                        [6]            
These soft symbols or estimates provide a path diversity L and a transmit diversity 2. Thus, the total diversity of the STTD system is 2L. This increased diversity is highly advantageous in providing a reduced bit error rate.
A problem with the 2L STTD diversity arises when L becomes too large. This is because the phase correction circuit (FIG. 3) of the mobile receiver must perform 2L complex multiply and 2L complex add operations for each symbol. The resulting complexity may substantially increase, for example, to L greater than 12 in a vehicular environment or during soft handoff when the mobile receiver moves between cells. These operations greatly increase processing complexity for the mobile receiver. Moreover, advantages of diversity diminish for values of L greater than 6 and may not improve reception.
These problems are resolved by a communication circuit, comprising a detector circuit coupled to receive a first signal. The detector circuit is arranged to produce a multipath signal corresponding to a number of paths of the first signal. A comparator circuit is coupled to receive the multipath signal and a threshold signal. The comparator circuit produces a diversity signal in response to a comparison of the multipath signal and the threshold signal. The diversity signal has a first logic state for enabling diversity and a second logic state for disabling diversity.
The present invention improves reception and reduces circuit complexity by providing a circuit at the mobile receiver to selectively enable and disable diversity. No additional transmit power or bandwidth is required. Power is balanced across multiple antennas.